Level II scour analysis for Bridge 22 (REDSVT01000022) on State Route 100, crossing the West Branch Deerfield River, Readsboro, Vermont

This report provides the results of a detailed Level II analysis of scour potential at structure
REDSVT01000022 on State Route 100 crossing the West Branch Deerfield River,
Readsboro, Vermont (figures 1–8). A Level II study is a basic engineering analysis of the
site, including a quantitative analysis of stream stability and scour (U.S. Department of
Transportation, 1993). Results of a Level I scour investigation also are included in
Appendix E of this report. A Level I investigation provides a qualitative geomorphic
characterization of the study site. Information on the bridge, gleaned from Vermont Agency
of Transportation (VTAOT) files, was compiled prior to conducting Level I and Level II
analyses and is found in Appendix D.
The site is in the Green Mountain section of the New England physiographic province in
southern Vermont. The 25.6-mi²
drainage area is in a predominantly rural and forested
basin. In the vicinity of the study site, the surface cover is forest.
In the study area, the West Branch Deerfield River has an incised, straight channel with a
slope of approximately 0.025 ft/ft, an average channel top width of 63 ft and an average
channel depth of 10 ft. The predominant channel bed materials are cobbles and boulders
with some bedrock exposure noted under the bridge. The bed material has a median grain
size (D₅₀) of 141.0 mm (0.463 ft). The geomorphic assessment at the time of the Level I and
Level II site visit on July 30, 1996, indicated that the reach was stable.
The State Route 100 crossing of the West Branch Deerfield River is a 119-ft-long, two-lane
bridge consisting of one 110-foot steel-beam span (Vermont Agency of Transportation,
written communication, September 28, 1995). The bridge is supported by vertical, concrete
abutments with spill-through embankments. The channel is skewed approximately 50
degrees to the opening while the opening-skew-to-roadway is 50 degrees.
The scour protection measure at the site was type-3 stone fill (less than 48 inches diameter)
on the spill-through embankments of each abutment and the banks upstream and
downstream. Additional details describing conditions at the site are included in the Level II
Summary and Appendices D and E.
Scour depths and rock rip-rap sizes were computed using the general guidelines described
in Hydraulic Engineering Circular 18 (Richardson and others, 1995). Total scour at a
highway crossing is comprised of three components: 1) long-term streambed degradation;
2) contraction scour (due to accelerated flow caused by a reduction in flow area at a bridge)
and; 3) local scour (caused by accelerated flow around piers and abutments). Total scour is
the sum of the three components. Equations are available to compute depths for contraction
and local scour and a summary of the results of these computations follows.
There was no predicted contraction scour for any of the modelled flows. Abutment scour
ranged from 4.9 to 11.6 ft. The worst-case abutment scour occurred at the right abutment for
the 500-year discharge. However, historical information indicates the right abutment is in
contact with bedrock at least in part. Additional information on scour depths and depths to
armoring are included in the section titled “Scour Results”. Scoured-streambed elevations,
based on the calculated scour depths, are presented in tables 1 and 2. A cross-section of the
scour computed at the bridge is presented in figure 8. Scour depths were calculated
assuming an infinite depth of erosive material and a homogeneous particle-size distribution.
It is generally accepted that the Froehlich equation (abutment scour) gives “excessively
conservative estimates of scour depths” (Richardson and others, 1995, p. 47). Usually,
computed scour depths are evaluated in combination with other information including (but
not limited to) historical performance during flood events, the geomorphic stability
assessment, existing scour protection measures, and the results of the hydraulic analyses.
Therefore, scour depths adopted by VTAOT may differ from the computed values
documented herein.